Draft: #168 : Utilize the new CI RDS database in CI package pipelines

General MR

Summary

This MR migrates the mattermost package away from using the builtin bitnami postgres module to utilising the new CI RDS database instance exposed by the pipeline.

Relevant logs/screenshots

Initial proof of concept work for this was shown on this clean install ci pipeline and this upgrade ci pipeline. Until the linked MR is merged, our pipelines won't succeed.

Linked Issue

#168

Upgrade Notices

N/A